Why Do Tesla Coils Fire Electric Arcs?

Resembling a death machine with its massive voltage and lightning discharge, the Tesla Coil was actually intended for free wireless electricity transmissions. It is a high-frequency air-core transformer capable of magnifying voltage beyond 1,000,000 units. The coil was pioneered by the genius Nikola Tesla (1856-1943) , a man whose numerous contributions include the AC induction motor, radio transmission before Marconi, the discovery of Earth’s resonance 50 years before it was proven, and hundreds of patents. Though Tesla Coils are now used only for education and entertainment, the devices are fantastic enough to inspire a slew of hobbyists into building their own, but how exactly does it work?

Inducing Oscillation
Nikola Tesla’s original device was a SSTC, or Spark Gap Tesla Coil, and is the simplest type of coil to construct if this is your first Tesla Coil project. Simply put, it is built from two inductive-capacitive oscillators that are loosely coupled together. These oscillators themselves consist of an inductor and a capacitor. The inductors are built from electrical conductors wound into coils. Their job is to convert electrical current into either a magnetic field or field current. Two main windings are used, called the primary and secondary winding. The number of coils determines the amount of voltage in each winding. Tesla Coils generally have less voltage in the primary winding than in the secondary output side for the purpose of creating immense voltage during the back-and-forth exchange.

Nikola Tesla casually reading a book under his Coil. Image courtesy of Teslasociety .

The capacitor portion of the oscillator converts electric current into electrical field or vice versa; it comprises of at least two conductors separated by an insulator. Once the capacitor is connected to an inductor, a magnetic field is then created as current flows between the two components. After the electric field in the capacitor is exhausted, the magnetic field will collapse and the current will cease. This induces the current to flow in the opposite direction back into the inductor. The capacitor is once again recharged by this new current resulting in an equal but opposite electrical field than the original. Oscillation will resume back and forth between the magnetic field and electric field, as the current continuously reverses, for as long the inductor and capacitor remain connected.

In layman’s terms
To illustrate this concept with an analogy, picture a bucket loaded with water being lowered halfway into a pond. As the bucket is tilted on its left side, the content begin to spew, but after the side of the bucket reaches the water level in the pond, it refills as pond water rushes in. Next the bucket is tilted in the opposite direction and the same exact thing occurs, just like the oscillating of the magnetic and electric field. Raising the bucket from the pond is akin to breaking the contact between the inductor and capacitor to cease the oscillation.

Spark Gap
Tesla Coils types are distinguished by the input voltage, the size of the components, and the switch used to connect the inductor to the capacitor. The classic Tesla Coil uses a spark gap as its switch, fundamentally a gap of air between the two conductors. Once the electric field stored in the capacitor reaches an appropriate threshold, the air within the space becomes ionized and closes the switch through the ensuing plasma that’s formed; this plasma is the visible “lightning” witnessed between the two oscillators of a traditional Tesla Coil.

The back-and-forth exchange occurs hundreds of thousands of times per second with a bit of extra energy being transferred to the second conductor during each exchange. As this occurs, the secondary circuit begins to resonate at the same frequency as the primary circuit and a vast voltage potential is created at the discharge terminal.

Conditions for maximizing the coil’s output
Two conditions must be met in order to maximize the output of a Tesla coil. First, the primary and secondary windings must oscillate at the same frequency, and second, the total length of conductor in the secondary winding must be equivalent to one quarter the length of the oscillator’s wavelength.

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Resource: users.tm.net